This invention relates to cutting tools, cutting tool inserts and wear parts. More particularly, it is concerned with monolithic or composite silicon nitride ceramic articles having an adherent refractory aluminum nitride coating deposited thereon.
Cemented carbide materials are well known for their unique combination of properties of hardness, strength, and wear resistance, and have accordingly found extensive use in mining tool bits, metal cutting and boring tools, metal drawing dies, wear resistant machine parts and the like. It is known that the wear resistance of cemented carbide materials may be enhanced by the application of thin coatings of a highly wear resistant material such as titanium carbide or aluminum oxide. These coated carbide materials are finding increasing commercial utility for certain cutting tool, machining, and wear part applications.
Economic pressures for higher productivity, for example in machining applications, are placing increasing demands upon the performance of such coated materials. To achieve high productivity in machining, a tool must be able to cut at high speeds. At cutting speeds exceeding 1500 surface feet per minute (sfpm), the high temperature strength and chemical inertness of a cutting tool material become increasingly important. The usefulness of cemented carbide cutting tool materials (the predominant material used in cutting tools) has been extended to applications requiring cutting speeds of about 1500 sfpm by coating such tools with aluminum oxide. For cutting speeds in excess of 1500 sfpm, cemented carbide tools encounter problems associated with coating adherence, loss of strength, and tool nose deformation, which affect dimensional tolerance in the workpiece and contribute to shorter tool life.
Conventional ceramic cutting tools overcome some of these disadvantages but have limitations relating to their lower impact strength and fracture toughness. This is especially true of many alumina-based conventional ceramic cutting tools. Silicon nitride-based ceramic cutting tools have significantly higher impact strength and fracture toughness, but can exhibit lower than desired chemical inertness when employed in cutting long-chipping metals such as steel. Improvements have been made by coating silicon nitride-based ceramic substrates with alumina or with carbides and/or nitrides and/or carbonitrides of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten, as disclosed and claimed in U.S. Pat. Nos. 4,406,667, 4,406,668, 4,406,669, 4,406,670, 4,409,003, 4,409,004, 4,416,670, 4,412,525, 4,412,528, 4,424,066, 4,426,209, 4,431,431, 4,440,547, 4,441,894, and 4,449,989, all commonly owned with the present invention. However, it would be desirable to improve even further the adherence and wear resistance of these substrates.
The present invention provides improved coated cutting tools and cutting tool inserts for the machining of metals under demanding conditions of machining speed, temperature, or workpiece hardness. The coated articles according to the present invention exhibit improved coating adherence and wear resistance, useful not only in cutting and machining applications, but also in wear part applications such as for metal drawing dies.